This invention relates in general to an apparatus for machining a workpiece by electro-erosion using a wire fed through a machining zone of the workpiece, and more specifically to an apparatus for controlling the gap between the workpiece and the outlet of a nozzle(s) delivering a machining liquid to the workpiece in accordance with the length and configuration of the groove being machined.
In such an apparatus the wire electrode is fed at a predetermined longitudinal velocity to compensate for the wear of the electrode, while being transversely displaced relative to the workpiece. The displacement between the wire electrode and the workpiece is generally effected automatically by a servo system under the control of programmed instructions defining a predetermined machining path.
In the conventional wire electrode discharge machining apparatus shown in FIGS. 1 through 4, reference numeral 1 designates a wire electrode supplied from a bobbin 2, 3 is a brake roller directly coupled to an electromagnetic brake actuator 3a to impart a predetermined tension to the wire electrode, and 4a, 4b and 4c are idlers for controlling the feed direction of the wire electrode. The wire electrode is wound around the brake roller 3 and the idler 4a, and the current applied to a coil of the brake actuator 3a maintains the wire electrode at a predetermined tension.
Upper and lower guides 5, 6 for the wire electrode are respectively disposed in upper and lower nozzles 7, 8 which deliver a machining liquid 10 under pressure to the upper and lower sides of a workpiece 12. A pump 9 supplies the machining liquid to the nozzles. The wire electrode 1 is supported by the upper and lower guides 5, 6, and extends through the workpiece 12 in a predetermined direction. A power supply 11 generates a periodic or pulsating discharge signal which is applied between the wire electrode and the workpiece. A pair of driven pinch rollers 13 wind in the exiting electrode wire at a predetermined velocity and deliver it to a catch bin. Reference numeral 14 designates machining particles or dusts which are melted and dispersed from the workpiece 12, and 15 is the groove being machined in the workpiece.
In such an apparatus the gap between the workpiece 12 and the outlets of the nozzles 7, 8 is initially adjusted or set by manually controlled servo motors 19a, 19b which drive frame members 21a, 21b fitted to the nozzles. Such gap is set at a predetermined width by adjusting both of the nozzles, or by adjusting one of them with the other one being fixed, and remains at such width throughout the machining operation. After this initial gap setting the power supply 11 is actuated to deliver a pulse voltage between the wire electrode and the workpiece, which in cooperation with the machining liquid 10 supplied from each nozzle coaxially with the electrode, machines the desired groove 15. The machining dust 14 which is melted and dispersed from the workpiece 12 by the heat energy of the periodic electrical discharges is exhausted to the outside through the groove 15 along with a stream of machining liquid 10, as shown in FIGS. 2 and 4. The machining liquid delivered from the nozzles divides into two streams, one stream C (arrow, FIG. 2) flowing through the groove 15, and another stream D striking the surface of the workpiece and dispersing.
The displacement between the wire electrode 1 and the workpiece 12, in order to maintain a small working gap between the electrode and the workpiece as shown in FIG. 3, is generally effected automatically by moving an X-Y table (not shown) via a servo system in accordance with programmed instructions in a numerical controller (conventional and not shown). The grooves 15 are thus machined continuously and in desired configurations by repeating the electrical discharges and appropriately controlling the X-Y table.
When the groove 15 being machined is straight as shown in FIGS. 3 and 4 and its rear side is open, the machining dust 14 is effectively entrained by and exhausted along with the liquid 10. When the rear side of the groove is closed, however, as shown at 15a in FIG. 5(a), or when the groove has a lateral arm 15b as shown in FIG. 5(b), the exhaust of the liquid 10 and machining dust 14 is impeded or even blocked. When the nozzle outlets do not completely cover or overlie the entire length of a "dead end" groove the machining liquid and dust can escape or exit from the exposed upper and lower portions of the groove, but when the groove is fully covered by the nozzles, such as at the beginning of the machining operation or at a right angle bend of the groove configuration, such exhaust is substantially blocked. This leads to machining inaccuracies and increased electrode wire consumption. Similar problems are encountered when the groove is sharply curved or spiralled as shown in FIGS. 5(c) and 5(d).